Aluminum DI cans and tinplate DI cans have heretofore been treated with separate special purpose surface treatment baths. Baths for treating the surface of aluminum DI cans are exemplified by the bath taught in Japanese Laid Open [Kokai or Unexamined] Patent Application Number Sho 52-131937 [131,937/1977]. The surface treatment bath taught therein is an acidic aqueous coating solution that has a pH of approximately 1.0 to 4.0 and that contains phosphate, fluoride, and zirconium and/or titanium. Treatment with this conversion treatment bath results in the formation of a highly paint-adherent, highly corrosion-resistant conversion film on the aluminum surface. The main components of this film are phosphate salt and zirconium oxide or titanium oxide.
Baths for treating the surface of tinplate DI cans are exemplified by the bath taught in Japanese Laid Open Patent Application Number Hei 1-100281 [100,281/1989]. This invention comprises a conversion film-forming bath for the treatment of metal surfaces. The bath in this case has a pH of 2 to 6 and contains I to 50 g/L of phosphate ions, 0.2 to 20.0 g/L of oxyacid ions, 0.01 to 5.0 g/L of tin ions, and 0.01 to 5.0 g/L of condensed phosphate ions. Treatment with this conversion treatment bath results in the formation of a highly corrosion-resistant film on the surface of tinplate DI cans. The main component of this film is tin phosphate.
A device known as a washer is generally used to treat the surface of DI cans. The formed DI cans are continuously treated with a degreasing composition and a conversion coating composition while turned upside down. Washers currently in use most often execute the following 6 processes: preliminary degreasing, degreasing, water wash, surface treatment, water wash, and a wash with de-ionized water.
In order to conduct surface treatment using such a washer, it has been necessary to select a surface treatment bath adapted to the type of DI cans to be treated. In other words, a treatment bath in accordance with, for example, the invention in Japanese Laid Open Patent Application Number Sho 52-131937 would be used as the surface treatment bath for aluminum DI cans, while a treatment bath in accordance with, for example, Japanese Laid Open Patent Application Number Hei 1-100281 would be used as the treatment bath for tinplate DI